The invention generally relates to motor control systems for controlling the operation of three-phase AC motors.
The invention more particularly relates to industrial applications of AC motors where quick response and a broad range of speed control are desired. One such application for AC motors is drive systems for elevators. Each time an elevator is stopped, the driving motor must be braked to a stop and then restarted to move the elevator to the next floor.
To develop torque in the motor driving the elevator, magnetic flux must be established in the rotor, the rotating element of the motor. To develop flux, current is supplied to the windings on the stator, which is the stationary portion of the motor formed around the rotor and separated from the rotor by an air gap. Flux is not established instantaneously but instead builds up over time as an exponential function of current. This time is related to a rotor time constant, which is the ratio (L.sub.R /R.sub.R) of rotor inductance L.sub.R to rotor resistance R.sub.R and the flux build-up time will be extended for a motor having a relatively higher time constant.
A prior method for starting an elevator drive motor included supplying the current that was required to maintain steady-state operating flux in the rotor. When this current was provided on startup, however, the response of the motor was not fast enough to provide suitable performance as an elevator drive.
A technical problem during startup of the motor control system in another application involved undesired movements of a rotor shaft due to small transient torques produced in the motor. It is believed that these small torques may be due to residual magnetism and hysteresis effects, or they may be due to the variation in the magnetic reluctance due to the configuration of slots in the stator or rotor. These small torques produced undesired movement of the rotor output shaft when the motor was not mechanically loaded.
A typical three-phased AC motor control system includes a power circuit and a control circuit. The power circuit usually includes an AC-to-DC converter, an intermediate DC circuit and an inverter for reconverting DC power to alternating currents that are supplied to the stator of the motor. The inverter includes a network of six semiconductor switches, which may be thyristors or power transistors. The currents that are supplied to the stator may be precisely controlled by controlling the firing or conduction of the semiconductor switches with one of several known control circuits. One type of control circuit uses pulse width modulation (PWM) to control the operation of the semiconductor switches.